Allow supplying org.slf4j.Marker to the logger

this fixes oshai#360

src/javaMain/kotlin/io/github/oshai/kotlinlogging/slf4j/Slf4jExtensions.kt

@@ -5,10 +5,17 @@ import io.github.oshai.kotlinlogging.KotlinLogging
 import io.github.oshai.kotlinlogging.Level
 import io.github.oshai.kotlinlogging.Marker
 import io.github.oshai.kotlinlogging.slf4j.internal.Slf4jLoggerFactory
+import io.github.oshai.kotlinlogging.slf4j.internal.Slf4jMarker
 import org.slf4j.Logger
 import org.slf4j.MarkerFactory
 
-public fun Marker.toSlf4j(): org.slf4j.Marker = MarkerFactory.getMarker(this.getName())
+public fun Marker.toSlf4j(): org.slf4j.Marker =
+  when (this) {
+    is Slf4jMarker -> marker
+    else -> MarkerFactory.getMarker(getName())
+  }
+
+public fun org.slf4j.Marker.toKotlinLogging(): Marker = Slf4jMarker(this)
 
 public fun Level.toSlf4j(): org.slf4j.event.Level =
   when (this) {

src/javaMain/kotlin/io/github/oshai/kotlinlogging/slf4j/internal/Slf4jMarker.kt

@@ -0,0 +1,5 @@
+package io.github.oshai.kotlinlogging.slf4j.internal
+
+import io.github.oshai.kotlinlogging.Marker
+
+internal class Slf4jMarker(val marker: org.slf4j.Marker) : Marker, org.slf4j.Marker by marker

src/jvmTest/kotlin/io/github/oshai/kotlinlogging/slf4j/Slf4jMarkerTest.kt

@@ -0,0 +1,75 @@
+package io.github.oshai.kotlinlogging.slf4j
+
+import io.github.oshai.kotlinlogging.KotlinLogging
+import io.github.oshai.kotlinlogging.addAppender
+import io.github.oshai.kotlinlogging.removeAppender
+import kotlin.test.assertEquals
+import org.apache.logging.log4j.core.Appender
+import org.apache.logging.log4j.core.LogEvent
+import org.apache.logging.log4j.core.appender.NullAppender
+import org.apache.logging.slf4j.Log4jMarker
+import org.junit.jupiter.api.AfterEach
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+
+private val logger = KotlinLogging.logger {}
+
+private class TestAppender(
+  val appender: Appender = NullAppender.createAppender("testAppender"),
+  var lastEvent: LogEvent? = null,
+) : Appender by appender {
+  override fun append(event: LogEvent) {
+    lastEvent = event.toImmutable()
+    appender.append(event)
+  }
+}
+
+class Slf4jMarkerTest {
+
+  private val testAppender = TestAppender()
+
+  @BeforeEach
+  fun setupAppender() {
+    addAppender(testAppender)
+  }
+
+  @AfterEach
+  fun removeAppender() {
+    removeAppender(testAppender)
+  }
+
+  @Test
+  fun `a slf4j Marker can be supplied to the logger`() {
+
+    val log4jMarker =
+      object : org.apache.logging.log4j.Marker {
+        override fun addParents(
+          vararg markers: org.apache.logging.log4j.Marker?
+        ): org.apache.logging.log4j.Marker = TODO("Not yet implemented")
+
+        override fun getName(): String = "foo"
+
+        override fun getParents(): Array<org.apache.logging.log4j.Marker> =
+          TODO("Not yet implemented")
+
+        override fun hasParents(): Boolean = false
+
+        override fun isInstanceOf(m: org.apache.logging.log4j.Marker?): Boolean = false
+
+        override fun isInstanceOf(name: String?): Boolean = false
+
+        override fun remove(marker: org.apache.logging.log4j.Marker?): Boolean =
+          TODO("Not yet implemented")
+
+        override fun setParents(
+          vararg markers: org.apache.logging.log4j.Marker?
+        ): org.apache.logging.log4j.Marker = TODO("Not yet implemented")
+      }
+
+    val slf4jMarker = Log4jMarker(null, log4jMarker)
+
+    logger.atError(slf4jMarker.toKotlinLogging()) { message = "bar" }
+
+    assertEquals(testAppender.lastEvent?.marker, log4jMarker)
+  }
+}